The present invention relates to a grinding machine for grinding ball end mills having helical cutter teeth.
One known grinding machine of the type described is a universal grinding machine with a turntable attachment. In operation, a ball end mill or milling cutter to be ground is fixed to a spindle of the turntable, and the turntable is turned to grind the spherical cutter teeth on the end of the milling cutter. The conventional grinding machine can grind the cutter end easily into complete spherical cutter teeth if the milling cutter being ground is a straight-tooth cutter called a slotting end mill having teeth on the circumerence parallel to the axis with zero rake angle. However, it is almost impossible for the known grinding machine to generate complete spherical cutter teeth if the teeth on the circumference are helical teeth having a large rake angle and a large helix angle of modern design for heavy-duty grinding. One practice, for example, has been to manually grind a thickness of material off an extension of the bottom of a face of the helical cutter teeth toward the axis of the milling cutter to thereby form a face in the vicinity of the tip of the spherical cutter teeth, fix the milling cutter to the spindle, and grind the spherical cutter teeth while turning the turntable. With this prior practice, however, only spherical cutter teeth similar in shape to an ellipsoidal surface of revolution can be generated dependent on the configuration of a flank grinding wheel, as described in another patent application entitled "Ball end mill with plain or tapered helical cutter teeth having a large helix angle and a large rake angle" filed by the inventor. According to another grinding practice devised by the inventor, the face of the spherical cutter teeth is first formed by a manual operation. Then, in a flank grinding process, a point guide having a height corresponding to an optimum face is positioned as closely as possible to the grinding point of the flank grinding wheel, and the tip of the face of the spherical cutter teeth is held against the point guide by rotating the spindle to keep the flank and the face in as fixed a relationship as possible to the grinding wheel while the milling cutter is being ground. However, this procedure has failed to meet fully with desired success because of the shape of the face and the position and shape of the point guide.
A couple of tool makers have recently employed a process in which manual grinding of the face of spherical cutter teeth is replaced with grinding of the face of spherical cutter teeth of a milling cutter about an axis passing through a point which lies in a plane formed by a line tangential to the face of a plain helical cutter tooth at its cutting edge at =0 and also by a cutting edge point of the spherical cutter teeth at the tip end thereof, which lies on a line normal to the bottom of the face, and whiel is equidistant from two points on the bottom of the face, the axis being perpendicular to the plane. With this process, however, the setting is approximate as described above and the face is a plane, so that the face will not be joined without involving a bent edge. Furthermore, in grinding a flank, spherical cutter teeth close to an accurate shape cannot even be generated without employing the process of the present invention. Since the face has a bent edge, it will leave a flaw on a workpiece surface cut thereby, making it difficult to effect a subsequent operation on the workpiece. The disadvantage with the point guide remains, so that no completely spherical cutter teeth can be generated. Recently available CNC tool grinding machines are pricipally the same as conventional tool grinding machines in mechanical construction and grinding principle, but differ therefrom only in that one-chucking heavy-duty grinding is made possible by employing a servo system for driving various shafts. Since the CNC tool grinding machine uses a computer, it requires a complex program and high-level technical skill, which cannot be provided by an person of ordinary skill in the art. Furthermore, the CNC tool grinding machine is quite expensive and has not found widespead use.